A DC-DC boost converter, such as for use, for example, in a power supply for LCD panels, typically has a power MOS transistor driven by a driver stage implemented in CMOS technology. The power MOS transistor is preferably also implemented in CMOS technology with the same integrated circuit and thus on the same chip. Accordingly, voltage limitations of the CMOS circuit likewise apply to the power CMOS transistor. The critical voltage is, of course, the admissible gate-source voltage. For a required output voltage of the converter, it is often necessary to operate the power MOS transistor at a supply voltage quite in excess of the maximum admissible gate-source voltage of the integrated power MOS transistor. Therefore, it is necessary to use a level shifter in the driver stage to move the gate-source voltage of the power MOS transistor to the right level.
Conventional solutions for such a level shifter require the use of large high voltage driver transistors. To protect the level shifter, a Zener diode is necessary. Both requirements add to the size and cost of the integrated circuit. In addition, since the value of the Zener diode voltage is equal to the minimum output voltage of the level shifter, such minimum value cannot arbitrarily be adjusted to the requirements of a particular application.